1. Field of the Invention
The present invention relates, generally, to an illumination system of a lithographic apparatus for fabricating a semiconductor device, and more particularly, to an illumination system using a diffractive optical element (DOE) that provides a multipole illumination.
2. Description of the Related Art
Generally, a lithographic apparatus having an illumination system is used to obtain optimized illumination conditions for forming a fine pattern on a semiconductor device. For example, FIG. 1 is a schematic diagram of a conventional illumination system including a conventional diffractive optical element (DOE). Referring to FIG. 1, a conventional illumination system 100 includes a conventional DOE 10. The DOE 10 divides a laser beam 5 projected from a light source (not shown) into several beams, determines the mutual angles of the divided beams, and provides a multipole illumination shape such as a quadrupole, dipole, or cross-pole. FIG, 2 illustrates a cross-sectional view of the conventional DOE in FIG. 1. Referring to FIG. 2, the DOE 10 is a non-spherical, light-diffracting device equipped with an uneven surface 13 formed with a predetermined pitch and depth on a surface of a base material 11.
The illumination system 100 produces the quadrupole illumination 15 by projecting the laser beam 5 through the DOE 10. An inner and outer sigma σ of the quadrupole illumination 15 are determined when passing through a zoom lens 20. Thereafter, the quadrupole illumination 15 is reflected by mirrors M1 and M2 and passes through a condenser lens 25, which condenses the quadrupole illumination 15. Then, the condensed quadrupole illumination is directed to a reticle 30 where a mask pattern of the reticle 30 is projected onto a wafer 40 by a projection lens 35.
The advantage of the lithographic apparatus having the above illumination system 100 is that light intensity does not decrease since an aperture, which blocks part of the light, for forming the multipole is not required. In addition, the poles may be enlarged or reduced, and the poles radial scope can be changed by the zoom lens 20.
However, once the illumination condition, that is the quadrupole, dipole, cross-pole, etc., is fixed by the DOE 10, a position of the poles in an angular scope and the relative sizes of the poles cannot be adjusted.
FIG. 3 illustrates the shape of the quadrupole illumination 15 produced by the DOE of FIG. 1. In this case, an illumination shape I(r, θ) can be expressed by multiplying a radial scope element A(r) by an angular scope element C(θ) where (r, θ) are polar coordinates. A(r) is 1 if rinner<r<router, otherwise A(r) is 0. C(θ) is 1 if b<θ<c, otherwise C(θ) is 0 (b and c are constants). The position of the poles in the angular scope is fixed at (b+c)/2 independent of rinner and router. The conventional quadrupole illumination 15 has four poles 15a, 15b, 15c, and 15d, each located in a different quadrant of the xy-plane and being symmetric about the x and y axes.
FIG. 4 shows a shape of a conventional cross-pole illumination 55 produced by another conventional DOE. In this case, an illumination shape I(r,θ) can be expressed by multiplying the radial scope element A(r) and the angular scope element C(θ), wherein (r,θ) are polar coordinates. A(r) is 1 only if rinner<r<router, otherwise A(r) is 0. C(θ) is 1 if 0<θ<b and d <θ<π/2, otherwise C(θ) is 0. A ratio of areas between the poles, b/(π/2−d)=1, is fixed. That is, angles between poles 55a, 55b, 55c, and 55d are each π/2, and these poles are located on the x and y axes.
As a consequence, the illumination shapes formed by the conventional DOEs depend on C(θ), which is only a function of θ, and has no dependence on r. Thus, even if the radial scope used is changed, the position of the poles in the angular scope and the relative sizes of the poles do not change. Accordingly, the conventional DOE has little pliability. Furthermore, in order to design the optimized illumination, each pole may be required to have a different size. However, it is very difficult to embody other illumination conditions by using the conventional DOE, which already embodies the optimized illumination condition.
Thus, problems in conventional DOEs include the small amount of pliability due to the fixed illumination mode, difficulty of changing a position and sizes of the poles, and difficulty of selecting and combining the illumination conditions.
Therefore, a need exists for a diffractive optical element (DOE) that produces a multipole illumination shape and is capable of changing a position of poles and sizes of the poles of the multipole illumination shape depending on a radial scope used.